1. Field of the Invention
The present invention relates to a thin-film deposition apparatus which forms a thin film on a semiconductor substrate, and, in particular, to a thin-film deposition apparatus which allows complete separation of a reaction chamber and a substrate transfer chamber.
2. Description of the Related Art
As semiconductor apparatuses have become highly integrated, ALCVD (Atomic Layer Chemical Vapor Deposition) having high thin-film deposition controllability attracts attention and replaces conventionally-used CVD (Chemical Vapor Deposition). Because ALCVD forms a thin film predominantly by absorption of atoms or molecules onto a semiconductor wafer, which is caused using multiple reaction gases to be absorbed onto the semiconductor wafer, thin-film deposition can deposit one atomic or molecular layer at a time and can form a film being several-molecules thick, and the step coverage of a resultant film is excellent.
When an ALCVD process is conducted, evacuating residual gas completely from a reactor before switching to a different reaction gas becomes important. If the residual gas remains inside the reactor, CVD reaction occurs in the vapor phase, making it difficult to control film thickness at the molecular layer level. Additionally, it causes a problem of generation of particles because a particle size generated becomes larger during vapor phase reaction.
To completely evacuate residual gas from within the reactor, conventionally a prolonged purge process was required; as a result, throughput was declined.
As a method for purging residual gas inside the reactor, the following method has been reported. That is, the reaction area and the transfer area are not completely separated and a slight gap (approximately 0.5 mm) is created; feeding an inert gas from the transfer area through the gap prevents process gas from entering into the transfer area (e.g., U.S. Pat. No. 4,854,263; and Japanese Patent Laid-open No. 2002-353207).
This method, however, was not able to completely prevent reaction gas from entering into the transfer area by diffusion; unwanted films deposited by CVD reaction were observed inside the transfer area. This indicates that the transfer area is at risk of particle generation because residual reaction gas cannot be completely evacuated except expediting a maintenance cycle. Additionally, it is difficult to shorten the purge time as well by this method.
Furthermore, in Japanese Patent Application No. 2001-361669, a method for separating a reaction chamber and a substrate transfer area by gas sealing is described. Because more processes use higher-frequency RF power as the wafer diameter has increased in recent years, impedance of a heater itself, which is embedded inside the susceptor, cannot be ignored in such processes. As a result, an electric potential difference between a grounded reactor and a susceptor becomes larger; if the electric potential difference exceeds the ionization potential of a gas used for sealing, plasma discharge is generated between the transfer chamber and the lower portion of the susceptor. In that case, a very small amount of deposition gas diffused from the reaction area causes a decomposition reaction in the plasma; accumulates adhere to a gate valve separating transfer chamber walls or a load-lock chamber and the transfer chamber. These accumulates can exfoliate and cause particle contamination.